Reassurance for mothers over mosaic aneuploidy in CVS

Professor Caroline Ogilvie's recent article ('Abnormal fetuses are highly unlikely to heal themselves', BioNews 847) is very helpful in reflecting the current state of understanding of mosaic aneuploidy in human embryo development.

Our recent findings about the fate of aneuploid cells in mosaic embryos (see BioNews 845) have indeed been inaccurately interpreted in the press in some cases, and those reports have occasionally included some very misleading statements.

I am in total agreement with Professor Ogilvie that our results do not in any way question the legitimacy of prenatal testing as it is currently employed. Early tests of fetal DNA in the mother’s blood or by chorionic villus sampling (CVS) are particularly important in assessing aneuploidy arising in meiosis where all of the embryonic cells are affected. These tests therefore represent an excellent means of spotting aneuploidy of the small chromosomes, such as chromosome 21, that can be tolerated but not without affecting the developing person.

Professor Ogilvie is also correct that we know that a normal baby can arise from a conceptus showing mosaic aneuploidy as detected by CVS. In our study we sought to find: first, why this should be so – what is the fate of those early anueploid cells; and second, what degree of mosaicism can be tolerated in the whole embryo (not just in the placenta). We showed for the first time that aneuploid cells are eliminated by programmed cell death (apoptosis) in the pluripotent cells that give rise to the fetus whereas they are tolerated in the extra-embryonic cells of the placenta where they divide more slowly.

We also gained an indication of the proportion of aneuploid cells that can be tolerated in the conceptus by varying the proportions of normal and aneuploid cells in experimental mouse embryos. This gives some insight into the numbers of normal pluripotent cells that are needed to make a viable fetus. The mouse embryo is similar to the human embryo in that at the blastocyst stage it usually has about 12 pluripotent epiblast cells out of a total of around 100 cells altogether.

One of our previous studies indicated that a minimum of four such pluripotent cells was required to make viable offspring in the mouse. Our new results are in broad agreement with this earlier finding.

Thus we believe our findings may constitute a strand of hope for mothers who have had early test results showing mosaic aneuploidy. Of course they should then have amniocentesis that will most likely show a normal karyotype. But they can also have some reassurance that although the amniocentesis cannot detect cells from all tissues of the fetus, experiments in mice show that cells with abnormal karyotypes are generally eliminated during early development.

Hopefully future studies will cast more light upon the underlying mechanism and at which stage of development these abnormal cells can be eliminated.